Mattis and Nyasani Watt, both 20-year-old men from Dorchester, are on trial, charged with the Sept. 25, 2011, killing of Blake, a middle school student who wrote poetry and dreamed of playing professional basketball.

In opening statements Tuesday, Suffolk prosecutors said Mattis and Watt were part of a gang called Flatline that was feuding with another group from Everton Street in Dorchester’s Bowdoin-Geneva neighborhood. They targeted Blake and the 14-year-old, prosecutors said, because they associated the younger victim’s address with their gang rival, a motive that police have tied to other killings of innocent teenage boys and men in certain crime-plagued neighborhoods.

“They wanted to send a deadly message,” Suffolk Assistant District Attorney Gregory Henning told the jury. “That message was about their toughness and their turf.”

Mattis and Watt, both dressed in button-down shirts and ties, expressed no emotion as Henning described the charges.

Lawyers for the suspects said prosecutors have no witnesses who can identify either man as the shooter. Willie Davis, Watt’s lawyer, said prosecutors have no witnesses who can put his client at the scene that day.

“They’re wrong,” Davis told the jury in his opening statement. “It will be up to you to tell them they’re wrong.”

Kelli Porges, Mattis’s lawyer, said that while it is true that her client was close to Watt and had “a friendly interaction” with Blake’s friend before the shooting, it is not enough to find Mattis guilty.

“That is the evidence the government is asking you to convict Sheldon Mattis for first-degree murder,” Porges told the jury. “That’s it.”

Henning told jurors that in the days leading up to the killing, Flatline gang members had been feeling anxious about their feud with the Everton Street group. Mattis had been beaten up by rivals and someone had fired at a house on Levant Street, where the leader of Flatline lived.

On Sept. 25, Blake and the 14-year-old had spent the afternoon playing video games when they decided to go to Walgreen’s for rolling papers to smoke marijuana, Henning said.

After Mattis spoke to the younger boy, he left to find Watt and helped him conceal a .40-caliber pistol, Henning said.

Mattis patted Watt on the back just before Watt took off on his bike down Geneva Avenue to find the boys, Henning said.

Watt allegedly fired six rounds at Blake and the 14-year-old, who was struck in the neck and shoulder, but managed to run away. Blake was shot through his spinal cord and collapsed to the ground, partially paralyzed and gasping for air, Henning said.

Boston police Detective Jesse Goff, the prosecution’s first witness, described for the jury Blake’s final moments.

Goff, then a patrolman, and another officer were patrolling the area when the shots were fired. They rushed to Geneva Avenue, where they found Blake lying facedown on the sidewalk. Goff searched for a pulse and felt nothing.

“I thought he was gone,” he said. But as the other officer did chest compressions, Blake suddenly sat up, looking terrified and confused.

“ ‘Get off me! Get off me!’ ” Goff recalled him yelling as he flailed his arms so wildly the paramedics had to handcuff one of his wrists to the gurney. “He didn’t know what was going on.”

That behavior is typical of someone fighting for survival, a paramedic who took the stand after Goff explained.

Blake died two hours later on the operating table at Boston Medical Center.

Henning promised jurors that they would be convinced of the defendants’s guilt by witness testimony, ballistics evidence, and footage from surveillance cameras that recorded that day’s events.

But Porges, Mattis’s lawyer, said prosecution witnesses will give inconsistent and contradictory statements.

“If you think one of the things they’re telling isn’t true, how do you accept anything they’re telling you?” she asked jurors.

Blake’s shooting echoed another killing in Bowdoin-Geneva, in 2010, when 14-year-old Nicholas Fomby-Davis was shot to death by two gang members who mistook him for a rival. In 2012, Joshua Fernandes, 19, and Crisostomo Lopes, 23, were convicted of the killing.

In July, 19-year-old Ricardo Arias was sentenced to life in prison for the 2011 killing of Alex Sierra, 18. Arias, a Mission Hill gang member, approached Sierra, who had no such ties, on a South End street, and asked him where he lived. When Sierra said Villa Victoria, a housing complex, Arias shot him multiple times.

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